The present invention relates generally to working of elongate webs of material, and in particular production of cuts in a web of plastic material. The invention is specifically, but not exclusively, aimed at production of plastic bags, especially plastic bags with handles.
A bag-making machine for producing plastic bags with handles is disclosed in EP-A2-0 847 851. In this bag-making machine, a tubular plastic web is passed through a number of working devices which by turns fold the web, provide the web with weld and perforation lines, provide the web with cuts forming the handles, and finally punch the web for removing web material cut out at the handles.
The cutting device which provides the web with handle cuts comprises a backing roll rotating counter-clockwise and a cutting roll rotating clockwise. The cutting roll has a cutting edge projecting from its circumferential surface and extending over essentially half the circumference of the cutting roll. The web is received between the backing roll and the cutting roll, and the cutting edge of the cutting roll abuts against the backing roll during part of each revolution to form a handle cut in the web. The radii of the backing roll and the cutting roll are selected in such a manner that their peripheral speeds correspond to the web feeding speed through the bag-making machine. In other words, the web moves a bag length for each revolution of the cutting roll and the backing roll. This prior-art bag-making machine is thus designed for production of plastic bags with a given bag length, and extensive modifications of the machine are necessary if longer or shorter bags are to be produced. Another problem of this machine is that there is a risk of cut-out web material coming loose even in the cutting device. Uncontrolled spreading of such material in the machine can result in operating troubles or machine breakdown.
Corresponding cutting devices are also known in envelope-producing machines and the like, such as U.S. Pat. No. 4,537,588,U.S. Pat. No. 4,599,926, U.S. Pat. No. 4,726,804 and U.S. Pat. No. 5,555,786.
An object of the invention is to wholly or partly obviate the above problems of prior art. More specifically, one object is to provide a method and a machine which are capable of making cuts in an arbitrary spaced-apart relationship in an elongate web of material.
A further object is to provide a method and a machine which allow a high production rate.
One more object is to permit continuous feeding of the web of material.
It is also an object to enable controlled removal of the cut-out material from the web.
Another object is to provide a simple technique of making continuous cuts as well as perforation lines in an elongate web of material.
These and other objects that will be evident from the following description are now at least partly achieved by a method for working an elongate web, preferably of plastic material, the web being fed at a given speed between a rotary backing roll and a rotary cutting roll which has a projecting cutting means, the cutting roll moving, during a first part of its revolution, the cutting means into engagement with the backing roll to form cuts in the web, and the cutting roll defining, during a second part of its revolution, with the backing roll a gap through which the web is moved essentially unimpededly, wherein the speed of rotation of the cutting roll is controlled relative to the speed of the web during the second part of the revolution in such manner that an optional distance is provided between succeeding cuts in the web.
The objects are also achieved by a machine for working an elongate web, preferably of plastic material, comprising a feeding device for feeding the web at a given speed through the machine and a cutting device for making cuts in the web, the cutting device comprising a rotary backing roll and a rotary cutting roll with a projecting cutting means, and the cutting roll being adapted to move, during a first part of its revolution, the cutting means into engagement with the backing roll to form the cuts and, during a second part of its revolution, define with the backing roll a gap through which the web moves essentially unimpededly, wherein a control means is associated with the cutting device and adapted to control the speed of rotation of the cutting roll relative to the speed of the web during the second part of the revolution for optional adjustment of the distance between succeeding cuts in the web.
The method and the machine according to the invention make it possible to form cuts in an essentially arbitrary spaced-apart relationship in a continuously supplied web of material. The peripheral speed of the cutting roll is suitably controlled to essentially correspond to the speed of the web during that part of the revolution in which the cutting means is engaged with the backing roll, while the cutting roll during the rest of the revolution is controlled at a peripheral speed which gives a desired distance between succeeding cuts in the web.
According to a preferred embodiment, the speed of rotation of the cutting roll is controlled in an infinitely variable manner relative to the speed of the web. Thus, it is possible to achieve any desired distance between succeeding cuts in the web. The speed of rotation of the cutting roll during the second part of the revolution can be increased for a reduced distance between succeeding cuts in the web and decreased for an increased distance between the cuts. The cutting roll is preferably driven by a first drive means with an infinitely variable number of revolutions, such as a servomotor.
It is also preferred for the backing roll to be given, with the aid of a second drive means, a peripheral speed which essentially corresponds to the speed of the web. As a result, undesirable tractive and elongation forces in the web are minimised.
According to another preferred embodiment, the second drive means comprises a drive roll which is made to abut against the circumferential surface of the backing roll. This enables easy adjustment of the distance between the backing roll and the cutting roll, for example for compensation for wear in the cutting means since the drive roll can be caused to follow the backing roll when displacing the same relative to the cutting roll. Moreover, a driven roll nip is formed between the drive roll and the backing roll and can be used for feeding the web of material.
According to one more preferred embodiment, a perforating device is controlled in conformity with the cutting roll to form perforation lines in the web. In this case, it is particularly preferred for the perforating device to comprise a perforation-forming cutting portion on the cutting means of the cutting roll. Thus, the perforation lines will automatically be in the desired position relative to the cuts, and besides the need for separate control of the perforating device is eliminated.
It is also preferred that cut-out material be removed from the web in a controlled fashion. According to an embodiment, a negative pressure is generated at least at one opening formed in the circumferential surface of the cutting roll for the purpose of retaining the cut-out material as the cutting means turns from the backing roll to a waste-receiving means, at which the cut-out material is delivered in a controlled fashion. Thanks to the cutout material thus being removed in direct connection with the cutting operation, the risk of uncontrolled spreading thereof is eliminated. The negative pressure is suitably generated in a portion of the circumferential surface of the cutting roll, said portion being enclosed by the cutting means.
Corresponding advantages also exist in preferred embodiments of the inventive machine.
According to another preferred embodiment, an element of a resilient material, preferably rubber material, is connected with said at least one opening in the circumferential surface of the cutting roll to form at least one mouth radially outside a cutting edge of the cutting means. The element is suitably adapted, when cooperating with the cutting roll, to be brought on a level with the cutting edge. This promotes removal of cut-out material from the web without any detrimental effect on the cutting operation. After cooperation with the backing roll, i.e. when the cutting means has cut out material from the web, the element in fact expands once more to its position radially outside the cutting edge. In this expansion, the peripheral speed of the element increases, whereby the cut-out material is torn loose from the web. The embodiment may be particularly advantageous in working of ductile materials, such as plastic materials.